Update .avatar_url to .display_avatar

[cgarz]

Fixes:

ERROR - Error on on_message: (<class 'AttributeError'>, AttributeError("'Member' object has no attribute 'avatar_url'")

Relevant info from discord.py discord's R. Danny bot:

1.x: https://discordpy.readthedocs.io/en/stable/api.html#discord.User.avatar_url

In 2.0, .avatar_url has been replaced with .avatar. https://discordpy.readthedocs.io/en/latest/api.html#discord.User.avatar.
1.x's .avatar has been moved to .avatar.key in 2.0.

.display_avatar has also been added to 2.0. This gets the avatar in the following order:

• Guild avatar
• User avatar
• Default avatar

This will ALWAYS return a Asset object, while .avatar can return both Asset and None if the user has no user avatar.